In the industry it is common to use pumps for pumping a fluid in a process plant such as a centrifugal pump used in an evaporator application. Such a pump typically has a casing in which an impeller is fitted. The impeller has a back side facing a back plate in the casing and a number of vanes on the front side in order to pump the fluid. Between the back side of the impeller and the back plate of the casing there is typically a gap. E.g. in evaporator applications where a fluid is heated and thus water is evaporated, a dry matter content of the fluid can become high. When a hot fluid with a high content of dry matter is processed a build up of the dry matter may occur in the casing. Especially in the mentioned gap between the casing and the impeller, the dry matter in the fluid tends to build up and deposit as a hard layer. This deposited dry matter creates an increased friction in the pump and thus a significant need for energy to power the pump. This energy is typically supplied from an electric motor. If the energy consumption becomes too high the motor will overheat and shut down. As a consequence a complete production system may shut down, and cleaning of the pump or of the complete system may have to take place before running the system again. Also, when the deposits are building up the efficiency of the pump is gradually reduced.
In e.g. a dairy plant it is common to have the plant working in two shifts, each of eight hours and performing cleaning in the third eight-hour shift, and in this way working round the clock. However, it is difficult to obtain efficient shift work as the pumps on the market tend to seize in deposits in shorter time.
To overcome the above mentioned problems it is known to have means for scraping between a back side of an impeller in a pump and a back plate of the pump. Such means have the form of scraper bars on back-sides of vanes on the impeller. The scraper bars per se are shaped as a kind of vanes on the back side of the impeller, and one vane-shaped scraper bar is arranged on each of the vanes. E.g. an impeller with six vanes would then have six scraper bars, one on each vane, which will have no influence on the balance of the impeller as all vanes and scraper bars have identical shape and are symmetrically arranged on the impeller. One such example is known from NL 275238, which document discloses one scraper bar on each vane. The scraper bars decrease building up of dry matter and thereby prolong an operational time of the pump, but in most cases the dry matter will still deposit faster than acceptable and thus lower a production time and capacity normally offered by the pump, while simultaneously increasing the energy consumption.